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Why athletes, outdoor workers, others should monitor the ‘wet bulb globe temperature’

The wet bulb globe temperature tells more of the story than the heat index does

Heat wave background
Heat wave background

ROANOKE, Va. – In recent days, I’ve been asked about the wet bulb globe temperature (WBGT). In all honesty, it’s not something I’ve paid attention to (or learned about) very much. (We learn about wet bulb temperatures in the winter, as that’s an important part of forecasting snow.)

After seeing multiple meteorologists in the Southeast post about it, I wanted to dive into why this could be a better temperature to use than the heat index. That’s especially for athletes, outdoor workers and the military.

According to the University of North Carolina, this temperature was developed in the 1950s by the military as a response to heat illness cases. It’s especially useful, because it takes more into account than the heat index.

It’s measured in the sun, uses temperatures, humidity, wind speed and sun intensity. The heat index only takes into account the temperature and humidity in the shade.

WBGT vs. Heat Index

It’s broken down into five different flags, based on what the wet bulb globe temperature is. The numbers and recommendations shown below are according to the North Carolina High School Athletics Association. Recommendations may differ by state and may differ from athletes to military personnel.


No Flag (less than 80°F) is coded in white, and indicates that there’s no present heat danger. Five-minute water breaks are recommended every 30 minutes.

WBGT - No Flag

A Green Flag (WBGT is between 80 and 84.9°) is used to indicate that athletes can practice normally. New or unconditioned athletes need to be monitored more closely during extreme exertion. Five-minute water breaks are recommended every 25 minutes.

Low flag WBGT

A Yellow Flag (WBGT 85-87.9°F) is used to show that new and unconditioned athletes should have a reduced intensity practice. Well-conditioned athletes should have more frequent breaks. Five-minute water breaks are recommended every 20 minutes, as is a cold water/ice pool on site.

Yellow Flag WBGT

A Red Flag (WBGT 88-89.9°F) means that all athletes need to be under constant watch. Pads and equipment should be removed. Five-minute water breaks are recommended every 15 minutes, as is a cold water/ice pool on site.

Red Flag WBGT

A Black Flag (WBGT of 90°+) means that practice should be suspended.

Black Flag WBGT

To get an hour-by-hour look of what the wet bulb globe temperature is, or is forecast to be, where you are, make sure to check out this link. You can also find more information on outdoor work by visiting this section of OSHA’s website.


About the Author:

Meteorologist Chris Michaels is an American Meteorological Society (AMS) Certified Broadcaster, forecasting weather conditions in southwest Virginia on WSLS 10 News from 5 a.m. to 7 a.m. weekdays on Virginia Today.